Preparation of L-Arginine-Modified Silica-Coated Magnetite Nanoparticles for Au(III) Adsorption

Introduction

In the era of modern technology today, gold is not only used for fixing, but also for  any application such as household appliances, electronics, cosmetics, biosensors and for the treatment of cancer cells^1,2. In industrial processes and manufacturing various electronic equipment,stages of coating, washing, chemical and mechanical polishingare necessary. At the stage of washing or rinsing waste water containing precious metal and hazardous ions is often generated³. Even thought the waste water produced only containing metal ions in low concentrations, those may be harmful to human health and other living creatures. Therefore those are necessary to be managed and removed from the waste water.

A variety of techniquesfor wastewater treatment and separation of precious metal ions have been reported, such as chemical precipitation⁴, ion exchange⁵, adsorption^6,7, membrane filtration⁸, and electrochemical^7,9. Among of the above techniques, adsorption has been widely used since it is proven to be more effective and economical¹⁰ because the design and operation are flexible.The adsorption is a reversibleprocess so thatthe adsorbent can be regenerated through desorption process forreuse¹¹.Efforts to develop the adsorbent containingselective functional groups have been conducted^12,13, but to recoverprecious metal ions and removal of heavy metal ion are still challenge. This is due to the capacity and selectivity towards targetedmetal ions low and high cost ¹⁴. Therefore, efforts to probe and develop a new adsorbent or method still continue.

The use of organic materials, such as organic polymers, cellulose, algae, and yeast biomass as adsorbent has aweakness, such as swelling, very sensitive to chemicals and lower its mechanical stability. In other hands, support inorganic solids, such as clay, zeolite, metal oxides and silica are preferable due to relatively resistant in term of physicochemistryand good selectivity. Silica gel has attracted the attention of researchers, because ofhigh chemical andmechanical stabilitiesand low swelling¹⁵. In addition, the silica containing a lot of silanolgroups on the surface makes easy to be modified with theorganic functional group.

Modification of silica gel with amine groups has been reported to be highly efficient in removing of Cu(II), Ni(II), Pb(II), Cd(II), Zn(II) and Hg(II) ions in aqueous medium^16,17. Aminoguanidinegroups have been used asa modifier of adsorbentfrom the tannin extract of persimmon for recovery ofAu(III) ion in acidic media¹⁸. Therefore, ligands with amine functional groups are very effective to adsorb precious metal ions¹⁹.Modification of the silica gel with the active site of mercapto (-SH) and an amine (NH₂) hasbeen reported by Nuryono et al.²⁰for Au(III) ion adsorption. The hybrid of amino-silica adsorbedtheAu(III) ion in similar amount (98.77%) tothatof mercapto-silica (96.67%) at the pH 4.In thatcondition, the amine groups of the amino-silica hybridsurface are protonated to form cationic groups-NH₃⁺ and interact with anionic species of Au(III) ion, AuCl₄^–. The interaction between Au(III) ion with amino-silica hybrid might happenthroughionic interactions.While the mercapto-silica hybrid, the S atoms donate the electron pair to Au(III) ions to form complexes using covalent interactions.

Nowadays, magnetic separation is considered as an effective method for the recoverypreciousmetalsand removal of heavy metal ions. Magnetite nanoparticles (Fe₃O₄) modified with functional groups such as dendrimer²¹, dimethylamine²², dimercaptosuccinicacid²³, and 3-aminopropyltrietoksisilan²⁴has been tested to be capable of removingheavy metal ions due to the high adsorption ability, time is short and separation of the adsorbent from the solution is easy because it uses an external magnetic field. The active site which contains a nitrogen or a sulfur atom  is  selectively high for the precious metal ion according to thehard soft acid base (HSAB) theory²⁵. The magnetite nanoparticles which modified by thiourea (SC(NH₂)₂)²⁶have been reported to be capable of separating and recovering precious metal ions from acid solution.

L-arginine molecules have aflexible chain of guanidine groups with pKa of 12.5.²⁷ Guanidine (-NHCNHNH₂) groups with apositivechargein acidic or neutral conditions leads to adsorb AuCl₄^– anion from species of Au(III) ion¹⁸.In this research, we have prepared L-arginine modified silica coated on magnetite nanoparticles surface using a linker of 3-glycidoxypropyltrimethoxysilane (GPTMS) in thesol-gel system.  The effectof amole ratio of L-arginine to GPTMS on the chemical stability, the content of nitrogen and the adsorption propertiesfor adsorption of Au(III) ions in aqueous solution has been evaluated.

Materials and Methods

Material

In this study,we used chemicals such asiron (II)/(III) oxide, 3-glycidoxypropyl-trimethoxysilane (GPTMS) (≥98%) from Sigma-Aldrich. Sodium silicate (25-27%), L-arginine powder, ethanol absolute (99.5%), HCl 37%, NaOH, HNO₃ 65%, thiourea, Au standard solutionwere purchased from Merck used without previous treatment. Precious metal of gold was purchased from PT. Aneka Tambang.

Preparation of L-Arginine-Modified Silica-Coated Magnetite Nanoparticles(Fe₃O₄/SiO₂-GPTMS-Arg)

In a beaker polyethylene, magnetite0.4 gwas added with 10 mLof distilled waterandsonicatedfor 5minutes. After that,the suspensionwas added12 mLof 2.8MNa₂SiO₃ solution andsonicated. After the suspensionstirred mechanically for 30minutes, it was added1.13mL ofGPTMSand stirred it about 30 minutes. Furthermore, the suspension was added with5mmol L-argininein order the mole ratio of GPTMS to L-arginine 1:1, stirredfor 60 minutesand 2MHCladdeddropwiseuntilpH7 and formed a gel. After the gelleft in one night, gelwashedwith aquamineraland flushed with ethanol three times, and dried at 65°C in the oven until its weight of constant.Analogue work was conductedvarying the mole ratioof GPTMS to L-arginine according to Table 1.

Characterization

The functional groupsinsamples wereidentifiedwith FTIR spectrophotometer (Shimadzu FTIR 8400S) with KBrpellettechnique in wave number400-4000 cm^-1. Powder XRD pattern of samples was compiled on Bruker D8 Advance 206276 (Sourceusedradiation  ofCu-Kα (λ 0.154 nm) on40kVvoltage and30mAcurrent. X-ray diffraction patterns were scanned at a2θrange of3-80°inscanspeed5°/min.The nitrogen content ofsamples wascounted with energy dispersive X-ray (EDX Carl Zeiss 9 EVO MA 10 series 1454)and Kjeldahl method. TGA was carried out on Mettler Toledo witharate of heating20°C per minute,at temperature 40-800°C. Absorption atomic spectrophotometer Analyst 100 Perkin Elmer apparatus was used to define thecontent of metal ion in solution. Niobium magnet was used for the magnetic separation. The measurement of thepH solution was conducted with a pH meter (pH/ion 510 EutechOakton).

The chemical stability wastestedby mixing25 mg of samplewith 25 mLHClsolution 1 M and shaking for3hours. After that, it was left overnightat a room temperature. The filtrate was dividedfrom the mixtureby an external magnetic force anddissolvedironwas determined by atomic absorption spectroscopy method^28,29.

Table 1: Mole ratio of GPTMS to L-arginine on preparation of the L-arginine modified silica-coated magnetite nanoparticles

 

Figure 1: Predicted reaction route of coating L-arginine modified silica on magnetite nanoparticles   Click here to View figure

 

Adsorption of Au(III)

Adsorption of Au(III) on Fe₃O₄/SiO₂-GPTMS-Arg was conducted by theeffect of pH, adsorption time and concentration of Au(III) ions.  Adsorbent 10 mg was addedto10 mL of Au(III) solution in a batch system ofa polyethylene bottle at a room temperature (302K). Themixturewas shaken at 350 rpm for certain time and the adsorbent was separated with an external magnet. The concentration of Au(III) ionnot adsorbed in thesupernatantwas measured by Atomic Absorption Spectrophotometer (AAS). A number of Au(III) ions adsorbed was calculated by the difference between the initial and final concentrationof Au(III) in the supernatant usingEq. (1)

q is Au(III) ions adsorbed (mmol.g^-1); C₀and C_e are initial and final concentrations of Au(III) (mmol.L^-1), respectively. V is volume (L) of the Au(III) solution and Wisthe adsorbent mass (g).

The pH of medium was controlledby0.1 M HCl and/or 0.1 M NaOH solution to reach 1-5, the effect of concentration of Au(III) ion was carried on 25-250 mg L^-1 and the variationof contact time was observed at the range of 5-180 min.  The models of Langmuir and Freundlich isotherm were applied to evaluate data of the effects ofinitial concentrationand the models of  somekinetics reaction are used to verify the appropriate adsorption rate constant.

The experiment of desorption was conducted by using thiourea solution in 1 M HCl- HNO₃. The firstly, 200 mg of adsorbent was added with 50 mL100 mg.L^-1Au(III) at optimum pH for 60 minutes. After the mixture was shakenand separated with a magnet, thesupernatant was analyzed by AAS to decide the content of Au(III) not absorbed. The adsorbentloadedwith Au(III)was washed byaquamineraland dried at 65°C to get theweight of constant and stored in a desiccator overnight before desorption process. The adsorbentloadedwith Au(III) was added with 10 mL of H₂O, and then was shaken for 10 min and supernatantwas separated with anexternal magnet. The supernatant was analyzed by AAS to decide thecontent of Au(III) that leached with H₂O. The adsorbent-loaded was added with 10 mL of 0.5 M thiourea solution in 1M HCl. Then the mixture was shakenfor 8 h andseparated with a magnet, andthe  Au(III) ion releasedwas measured its concentration. The desorptionwas repeated three times using the same eluent. Desorption data of Au(III) was calculated with Eq 2³⁰:

Results and Discussion

Characteristic of Fe₃O₄/SiO₂-GPTMS-Arg

Functional groups

The FT-IR spectra of silica-coated magnetite nanoparticles with and without L-arginine modification are showed in Fig 2. All spectra have a peak at 568.96 cm^-1. The peak is ascribed to Fe-O mode of magnetite³¹. Peaks3446.56 cm⁻¹ and 1645.17 cm⁻¹ are associated the O-H stretching and bending vibration, respectively (Fig 2a).The characteristic peak of silica is observed ataround of 1100 cm⁻¹related to the Si-O(Si-O-Si)asymmetricstretching vibration³². In all spectra, the peak at 796.55 cm^-1 is observed, assigned to symmetric stretching of siloxane Si-O(Si-O-Si), while the band around 460.96cm⁻¹ corresponds to the Si-O-Si or O-Si-O bending modes³³. This suggests that silica has been coated on the magnetite surface³⁴. FromFig 2(b-d) can be seen peaks at 3415.70, 3443.06 and 3411.84 cm^-1 ascribing the presence of amino groups. Those peaks are apparentbroader than that of Fig 2a due to overlappingwith O-H stretching vibration.

Figure 2: FTIR spectra of  Fe3O4/SiO2/GPTMS-Arg 1:0 (a), Fe3O4/SiO2-GPTMS-Arg 1:1 (b), Fe3O4/SiO2-GPTMS-Arg1:2 (c), and Fe3O4/SiO2-GPTMS-Arg 1:3 (d) Click here to View figure

 

Peak around 2939.31 cm^-1is appointedto thestretching vibration of C-H asymmetric³², fromGPTMS and L-arginine. The peak at 1645.17 cm⁻¹ (Fig 2a) is related the O-H (Si-OH) bending vibration (due to condensation of GPTMS), while the peak at 1654.81, 1662.52, and 1656.74 cm⁻¹ (Fig 2b-d)are related to the N-H bending vibration^18,35. It indicates that the coupling agent of GPTMS and modifier of L-arginine have been modified on silica-coatedmagnetite nanoparticles surface.

Crystalline Structure

The X-ray diffraction patternof the L-arginine modified silica coating on magnetite nanoparticles surface synthesized with various mole ratios of GPTMS to L-arginineare presented in Fig. 3. The observationof X-ray diffraction pattern for four samples seemscharacteristic peaks of magnetite confirmed byJCPDS 19-0629with the index field (220), (311), (400), (511) and (440)³⁶, and hence it denote that the cubic crystal phase of magnetite is stand after coating with L-arginine-modified silica³⁷. The presence of SiO₂amorphous phase is showed by specific 2θ at 22 degrees³⁸(Fig.3b, c, d, and e) according to JCPDS No. 46-1045.

Figure 3: XRD patterns ofFe3O4/SiO2-GPTMS-Arg 1:0 (a) Fe3O4/SiO2-GPTMS-Arg1:1(b),Fe3O4/SiO2-GPTMS-Arg1:2(c), and Fe3O4/SiO2-GPTMS-Arg 1:3 (d)  Click here to View figure

 

Chemical stability

Magnetite powder is readilydissolvedin acidic solution³⁹. Silica coating may stabilize magnetite from an acidic medium^34,39,40. In this work, the magnetite nanoparticles werecoated with silica and added amodifier of L-arginine. The samples of Fe₃O₄/SiO₂-GPTMS-Arg obtained were mixed with hydrochloric acid 1 M for 1day. Iron leachedfrom the sample was determined with AAS. The result showed that the acid solution leached iron from the coated magnetite samples in a range of 0.235-0.344 mmol g^-1. It is much lower than the amount of iron leached from uncoated magnetite (1.09 mmol g^-1).

The content of nitrogen element

L-arginine as a modifier to thesilica coated magnetite contains amine groups which may react wellwith Au(III) in solution.Based on the FTIR spectra in Fig. 2 seem that coated magnetite contains amine group -NH from L-arginine.In addition, we also confirmed the presence of nitrogen element from L-arginine usingthe EDX and Kjeldahl methods⁴¹ and the result is expressed in Table 4. The content of nitrogen in sample synthesized with amole ratioof GPTMS to Arginine 1:2 and 1: 3 analyzed with EDX and Kjeldahl methodsare relativelysimilar and higher than in sample synthesized with the ratio 1:1. However, the content of nitrogen based on the theoretical calculation is much greater than result from theexperiment. This difference occurs probablysince not all arginines are bonded on the surface. The presence of nitrogen in all samples supports the success of modification of arginine on silica coated magnetite.

Table 4: The content ofnitrogen inL-arginine modified silica-coated magnetite nanoparticles based on measurements of EDX, Kjeldahl method, and the theoretical calculation

 

Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was conducted on four samples of magnetite coated with silica-arginineat temperaturesfrom 40°C- 800°C, with the heating rate of 20°C per minute. Fig.4 shows that there are four stagesof weight lossin the TGA curve. The first stage, degradation occurs at a temperature of 40-100°C, the second stage of 100-250°C, the third stage of 250-500°C, and the fourth stage of 500-800°C. In Fig. 4 is observed that all four samples of coated magnetite lostthe weight at atemperature of 40-100 °C (<100 °C).The weight loss is related to the removal of water molecules adsorbed physically, which removal of water molecules is continued at the temperature of 100-220°C ^42,43. Fig.4 shows that the weight lossin the second stage at that temperature range (100-250°C), for Fe₃O₄/SiO₂-GPTMS-Arg 1:0, 1:1, 1:2, 1:3is 3.279; 3.384; 4.306 and 4.707%, respectively.

At the temperature range of 250-500°C, TGA curves of Fe₃O₄/SiO₂-GPTMS-Arg 1:0 the weight loss is 9.208%. The weight loss is linked to the decay of the organic part of a silica network, as reported Shajesh et al.⁴³. Another reason of the weight loss is a dehydration and dehydroxylation reaction of silanol (vicinal, geminal, and combinations) in the silica^42,44. Meanwhile, Fe₃O₄/SiO₂-GPTMS-Arg 1:1, 1:2, and 1:3 at the temperature range of 250-500°C loosedthe weightof7.570, 18.549, and 17.577%, respectively. In that temperature, the decomposition of organic parts^43,45, including L-arginine bound to the silica networkproduces CO and CH₄. At higher temperature than 500°C NH₃ is released. At atemperature rangeof 250-500 °Cshowthat the percentage of the weight loss is higher for L-arginine-modified silica-coated magnetite sample preparedwith greatermole ratio. This phenomenon can be explained that smaller size nanoparticleshave thelarger surface areaso that it can bind arginine on the surface in larger number, and the percentage of weight loss is greater^46,47.

At atemperature rangeof 500-800 °C (inthe fourth stage),the weight loss is related to the silanol dehydroxylation, especially silanol ingeminal position⁴². Thermal decomposition of all sample at this temperature are decreasing, this shows that all the decomposition stopped. In addition, at temperature 500°C, the magnetiteis changed into hematite⁴⁸.

Figure 4: Thermogravimetry analysis curve of Fe3O4/SiO2/GPTMS-Arg 1:0 (a), Fe3O4/SiO2-GPTMS-Arg 1:1 (b), Fe3O4/SiO2-GPTMS-Arg 1:2(c), and Fe3O4/SiO2-GPTMS-Arg 1:3(d). Click here to View figure

 

Adsorption properties

Effect of medium pH on the Au(III) ion adsorption

The experimental resultto study the effect of pHon is presented in Fig.5 showing that for all investigated adsorbents adsorption of Au(III) reaches optimum at pH 3. In the highly acidic condition (at pH 1-2), the adsorption is very lowand at higher pH (above 3), the adsorptiondecreases. AdsorbentsFe₃O₄/SiO₂-GPTMS-Arg 1:1, 1:2, and 1:3 show relatively similar adsorption capability (61.95, 67.45, and 66.88 mg g^-1,respectively). Fe₃O₄/SiO₂-GPTMS-Arg 1:0 gives very low adsorption capability. It is associated with theactive site in the magnetic adsorbent surface, without L-arginine,the probable active site playing a role in adsorption is only -OH of GPTMS. Au(III) in solution at pH 2-3 is dominant as a stable complex ion [AuCl₄]^–and at increased pH, chloride ion in the complex [AuCl₄]^–  is replaced by OH^– ions.  At a pH of 4-9 in [AuCl₃OH]^–[AuCl₂OH₂]^–[AuClOH₃]^–are formed and at pH higher than 9 the [AuOH₄]^–form dominates⁴⁹.In the acidic condition amino groups (NH₂) of arginine on silica-coated magnetite nanoparticlessurface undergo protonation to generate positively charged groups(-NH₃⁺).Those groups mayinteracts electrostatically with AuCl₄^–, a species of Au(III) in acidity. As seen in Fig. 5 the adsorption of Au(III) at pH 3 is observedmaximum.The interactionbetween thesurface of Fe₃O₄/SiO₂-GPTMS-ArgandAu(III) metal ionsin acidic conditions is illustrated hypothetically in Fig.6.

Figure 5: Effect of pH on Au(III) ion adsorption by Fe3O4/SiO2-GPTMS-Arg 1:1(a), Fe3O4/SiO2-GPTMS-Arg 1:2 (b), and Fe3O4/SiO2-GPTMS-Arg 1:3 (c). Click here to View figure

Figure 6: Hypothetic reaction on the formation of ionic bonds between Fe3O4/SiO2-GPTMS-Argwith Au(III)ion in aqueous solution.  Click here to View figure

 

Adsorption isotherm of Au(III)

The adsorption isotherms of Au(III) on four magnetic adsorbentswere tested withthe effect ofinitial concentrations of Au(III) (25-250 mg/L) at pH 3. The adsorption experimental data were evaluated by Langmuir and Freundlich isotherm models^34,50 expressed in mathematical equation (Eq.3and 4).

where q_e and C_e are the amount Au(III) adsorbed and concentration of Au(III) at equilibrium, q_m is adsorption capacity and K_L is the constant of Langmuir relating tothe binding sitesaffinity. The value of q_m obtained from slope and K_L found from the intercept of a plotof C_e/q_evsC_e.K_F.The constantof Freundlich is related tothe adsorptioncapacity and n is Freundlich exponent linked to the intensity of adsorption. The values of K_F and nare obtained from slope and intercept of plot log q_e vs log C_e. The adsorption process according to Langmuir isotherm isassumed as monolayer and occurs on the sites of the homogenous surface. While the Freundlich isotherm assumed that the adsorptionis heldon the sites of the heterogeneous surface. The adsorption data of Au(III) on Fe₃O₄/SiO₂-GPTMS-Arg 1:0-1:3 are shown in Fig 7 and Table 5 presents the Langmuir and Freundlich isotherm constants for adsorption of Au(III) on Fe₃O₄/SiO₂-GPTMS-Arg 1:0-1:3. The linear coefficient (R²) values for Langmuir isotherm models, in general, are higher than 0.99. This indicates that the Langmuir isotherm fits for the experimental data of Au(III) adsorption.

From Table 5 can be seen that the value of the adsorption capacity of Fe₃O₄/SiO₂-GPTMS-Argincreases with the molenumber of  L-arginine added, namely 0.209; 0.544; 0.638; 0.654 mmol/g, which correspondto the nitrogen content.Thisindicates thatfunctional group participates in the adsorptionis–NH₃⁺.The adsorbent without L-arginine (Fe₃O₄/SiO₂-GPTMS-Arg 1:0) giveslowest adsorption capacityand the adsorbentscontaining L-arginine showcapacity 1.87-3.13 times higher. The value of DG° (Table 5) are higher than 21 kJ/mol⁵¹indicating the involvement of chemical interaction, asillustrated in hypothetic reaction(Fig 6).

Adsorption kinetics

Study on the kinetics of Au(III) adsorption was conducted by interacting with 50 mg L^-1 Au(III) solution at various contact times (5-180 min),  pH 3 and room temperature (302 K). The adsorption data is presented in Fig. 8. It is observed that in the first five minutes adsorption occurs rapidly, followed by slow inclining and lastly reached equilibrium after 60 minutes.  It was associated with a lot number of sites on the coated magnetite adsorbent surface available for Au(III)in the earlyreactionand finalized with equilibrium after saturation.

Figure 7: Adsorption isotherm of Au(III) on Fe3O4/SiO2-GPTMS-Arg 1:0 (a), Fe3O4/SiO2-GPTMS-Arg 1:1 (b), Fe3O4/SiO2-GPTMS-Arg 1:2 (c), Fe3O4/SiO2-GPTMS-Arg 1:3 (d). Click here to View figure

 

Table 5: Adsorption isotherm parameters of Au(III) on Fe₃O₄/SiO₂-GPTMS-Arg

 

The reaction kinetics models of pseudo-first-order and pseudo-second-order were applied to evaluate the adsorption of Au (III) on the adsorbents, two common models for adsorption of metal ions on asolid adsorbent. The reaction kinetics models of pseudo-first-order and pseudo-second-order are stated inEq (5) and (6), respectively^52,53.

q_eand q_tare the amounts of adsorbed metal ion at equilibrium and anytime, respectively (mmol. g^-1), and k₁(min^-1) and k₂ (g.mmol^-1.min^-1) are the rate constants of pseudo-first-order and pseudo-second-order. The rate constants can be determined from the linear curve plot of  (ln q_e-q_t)against t andt/q_t against t to pseudo-first order and pseudo-second order reactions, respectively. The result of calculation is presented in Table 6, showing that the adsorption kinetics model of pseudo-secondorderis more suitablewith the higher linear coefficient(R²) value (0.999) than pseudo-first-order one. This shows that the adsorption system involves the exchange of electrons between the magnetic adsorbent and Au(III) ion in aqueous solution^37,53.

Figure 8: Effect of time on Au(III) ion adsorption on Fe3O4/SiO2-GPTMS-Arg 1:0 (a), Fe3O4/SiO2-GPTMS-Arg 1:1(b), Fe3O4/SiO2-GPTMS-Arg 1:2 (c), Fe3O4/SiO2-GPTMS-Arg 1:3 (d). Click here to View figure

 

Table 6: Adsorption rate constant of Au(III) on adsorbents based on the kinetics modelsof pseudo-first  and pseudo-secondorder

 

Desorption

The Au(III) ion adsorbed on Fe₃O₄/SiO₂-GPTMS-Arg was recovered by sequence desorption experiments. Desorption of precious metalsby using a thiourea solution, HCl, thiourea-HCl, HNO₃,thiourea-HNO₃has  been reported by previous researchers^{22,26,30,54,55}. In this work, desorption of Au(III) adsorbed onFe₃O₄/SiO₂-GPTMS-Arg 1:2(25.068 mg/g) was carried outwithin the different composition of eluent at a room temperature as presented in Table 7.Each fraction (10 mL) of desorption, the concentration of Au(III) desorbed was determined with AAS.  As shown in Table 7, desorption using1 M thiourea-0.5 M HClgives the highest desorption percentage (75.2997%). However, this result has not satisfied yetand theinvestigation is still going on to reach 100 % of recovery.

Table 7: The desorption data ofAu(III) loaded onFe₃O₄/SiO₂-GPTMS-Arg 1:2with different eluents.

Desorber eluent	Fraction/Eluent	Au(III) desorbed (%)	Desorption Total (%)
0.5 M Thiourea-1 M HCl	10 mL Aquademineral	0.5231	60.8431
	10 mLThiourea -HCl	57.8411
	10 mL Thiourea -HCl	2.4789
	10 mL Thiourea -HCl	0
1 M Thiourea-0.5 M HCl	10 mL Aquademineral	0	75.2997
	10 mLThiourea -HCl	54.7049
	10 mLThiourea -HCl	20.4661
	10 mLThiourea -HCl	0.1287
0.5 M Thiourea-1M HCl-1M HNO3 (3:1)	10 ml Aquademineral	0	64.3061
	10 mLThiourea – HCl-HNO3	58.4736
	10 mLThiourea – HCl-HNO3	5.6993
	10 mLThiourea – HCl-HNO3	0.1332

 

Conclusion

In conclusion, L-arginine-modified silica-coated magnetite nanoparticles (Fe₃O₄/SiO₂-GPTMS-Arg) have been successfully synthesized using 3-glycidoxypropyltrimethoxysilane as the coupling agent via a sol-gel process with the optimum mole ratioof GPTMS to arginine of 1:2.Adsorption of Au(III) on Fe₃O₄/SiO₂-GPTMS-Arg 1:2 occurredoptimally at pH 3, followed themodelof pseudo-second-orderwith arate constant of1.77 g. mmol^-1.min^-1 and fitted to themodel of Langmuir isotherm with the capacity of 0.64mmol.g^-1.The further investigation to findthe best desorption technique in which Au(III) may be leached from adsorbent 100 % is still going on, and it is expected that magnetic material produced may be useful for recovery of precious metals from both industrial waste water and precious metal mining samples.

Acknowledgement

We would like to give the highest gratitude to the Ministry of Research, Technology and Higher Education of the Republic of Indonesia through the Research Grant Disertasi Doktor2016 and Postgraduate Scholarship for the financial support.

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